Recording elements for electrostatic printing



United States Patent 3,378,370 RECORDING ELEMENTS FOR ELECTROSTATIC PRINTING Sebastian T. Branc'ato, Queens Village, N.Y., assignor to Interchemical Corporation, New York, N.Y., a corporation of Ohio No Drawing. Filed Feb. 6, 1964, Ser. No. 343,152

6 Claims. (Cl. 961.8)

ABSTRACT OF THE DISCLOSURE A recording element for electrostatic-printing wherein the photoconductive coating contains zinc oxide dispersed in a film of two partially incompatible resins.

This invention relates to electrostatic printing and particularly to improved recording elements for electrostatic printing and methods of producing these recording elements.

An electrostatic printing process is a process for producing a visible image or copy by converting a light image or signal into an electrostatic charge pattern on an electrically insulating base. The charge pattern is then converted or developed into a visible image by the application thereto of electrically-attractable particles. A typical electrostatic printing proces utilizing a photoconducting insulating stratum includes first producing a blanket electrostatic charge on the surface of the photoconducting stratum. The electrostatic charge may be stored on the surface for a time in the dark. The rate at which this stored electric charge is dissipated when the stratum is maintained in darkness is referred to as the dark decay of the stratum. Within the period in which a substantial charge remains, a light image is focused on the charged surface, discharging the portions of the surface irradiated with light, leaving the remainder of the surface in a charged condition, and thereby forming an electrostatic image thereon. The electrostatic image is rendered visible by applying to the electrostatic image a developer substance, such as a pigmented thermolastic resin powder, which is held electrostatically to the charged areas of the surface. The visible image thus formed may be fixed directly to the surface, for example, by fusing the image thereto.

A proces of electrostatic printing in which the thermoplastic resin powder is fused directly to the surface of the stratum is described in US. Patents 3,052,539 and 3,052,540, both to H. G. Greig.

The recording elements of this invention may be most effectively used in the process and .apparatus described in the Greig patents and represent improvements of the recording elements described in the Greig patents.

The recording element used in the process of the Greig patents comprises a backing sheet, such as paper coated with photoconductive white zinc oxide particles of high purity suspended in an electrically-insulating film-forming binder. The photoconductive zinc oxide preferably has a surface photoconductivity higher than ohms square/watt/cm. when exposed to a Wavelength of about 3900 A. and is very pure, e.g., zinc oxide C.P. or U.S.P. The electrically insulating film may be of a wide variety of substances. Synthetic resinous materials having high dielectric strengths are most desirable. These are known in the art as dielectric resins. Examples are polyvinyl acetate, copolymers of vinyl chloride-vinyl acetate, polystyrene, styrene-butadiene copolymer resins, and silicone resins. Other resin-like materials such as cellulose ethers and esters as well as natural resins may also be used.

In addition, as set forth in Patent No. 3,052,540, the

3,378,370 Patented Apr. 16, 1968 coating of the recording element may further include one or more organic dyes which are capable of absorbing radiant energy in bands of wave lengths to which said zinc oxide particles are relatively insensitive and transferring said absorbed energy to said zinc oxide particles, thereby substantially increasing the total sensitivity of the photoconductive coating to light.

In the electrostatic printing art, there is a constant need for recording elements of increased light sensitivity of such as that described in Greig Patent No. 3,052,539; this increased light sensitivity in the recording elements cuts down on the amount of exposure time necessary and thereby permits more rapid copying. In addition, the increased light sensitivity permits the use of a light source of lesser intensity in the apparatus thereby lessening the possibility of overheating, particularly when the apparatus is relatively compact.

It should be noted that by light sensitivity, we mean the tendency of the charged photoconductive surface to discharge in the areas on said surface irradiated by a light image focused on said surface. Accordingly, a photoconductive surface of increased light sensitivity will display an increased rate of discharge in the irradiated areas. This may also be termed an increased light decay rate.

I have now discovered an improved recording element having a photoconductive surface layer of significantly increased light sensitivity and increased light decay rate. We have found that by using a partially incompatible blend of two resins for the film forming binder in an otherwise conventional recording element of a base sheet carrying a photoconductive layer of said binder and a finely divided photoconductor such as zinc oxide dispersed in said binder, the light decay rate of the photoconductive layer is significantly increased, e.g., from twoto four-fold, over photoconductive layers containing either of the two component resins alone as the binder Without affecting any other properties of the recording element.

This result is surprising and unexpected because any existing teaching which would suggest the use of blends in the binder emphasizes that these blends must be fully compatible.

By partially incompatible, I mean that the resins in the proportions blended must be sufliciently incompatible to produce a haze when deposited as a dry film formed from a solution of the blend. However, they must not be so incompatible that macroscopic discontinuities are present in the film.

The resins which are blended together may beaosen from the wide variety of resins conventionally used as the sole binder resins in electrostatic recording elements. Such resins are generally described in US. Patents 3,052,539 and 3,052,340. The resins known as dielectric resins have high dielectric strengths. Among these resins are included acrylic ester polymers, rubber resins, polyester resins, polyvinyl acetate, copolymers of vinyl chloride-vinyl acetate, polystyrene, silicone resins, cellulose ethers, cellulose esters and shellac. Other suitable resins as Well as some particularly desirable blends for the practice of this invention will be discussed in greater detail hereinafter.

I have devised a simple procedure for determining Whether the two resins in the proportions in which they are to be blended meet the definition of partially incompatible. The two resins in said proportions are dissolved in a common solvent for both resins and the solution is deposited on a clear 46 mils thick glass slide and dried to produce a dry film, 1 mil in thickness. Then using the G.E. Spectrophotometer both the percentage of total light which is transmitted through the coated slide and the percentage of total light which is scattered are determined and percent haze is calculated according to the following formula:

Percent total light scattered Percent total light transmitted The percent haze is also calculated for the uncoated glass slide and the resulting percent haze is substracted from the percent haze .of the coated slide to give the actual percent haze of the film. Hereafter in this specification and claims, the percent haze given will be the corrected or actual percent haze of the film. Next, similar coated slides are prepared in which each of the two resins of the blend is used alone in an amount equal to total weight of the blend and percent haze is determined in each case. The film of partially incompatible blend will show a higher haze percentage than films in which the component resins are used alone. Also, the film of the blend should appear to be continuous to the naked eye. There should be an absence of macroscopic discontinuities in the film. In almost all cases, the haze on the slide containing the blend will be visible to the naked eye, giving the coating a milky white or frosted appearance. The haze percentage of dry films of the blend are preferably greater than 1% and less than 48%. At haze percentages over 48%, macroscopic discontinuities begin to appear in many cases.

It will be obvious to those skilled in the art that even where blends of two resins in certain proportions do not give partial incompatible blends as defined herein, blends of the same resins in other proportions may produce such partial incompatibility.

There are substantially no limits on the proportions in which the resins must be present in the blend .other than those necessary to meet the test for partial incompatibility set forth above.

While the photoconductor is preferably zinc oxide of the type described in U.S. Patent 3,052,539, other finely divided conventional photoconductors may be used such as the oxides of antimony, aluminum, bismuth, cadmium, mercury, molybdenum and lead; the iodides, selenides, sulfides or tellurides of these metals including zinc; selenium; arsenic trisulfide; lead chromate and cadmium arsenide.

The coating may also contain other conventional additives such as the waxes e.g., parafiin and carnauba and the plasticizers described in U.S. Patent 3,052,539 as well as sensitizing dyes such as those described in U.S. Patent 3,052,539.

It should be noted that unless otherwise stated, all proportions in this specification and claims are by weight.

I have found that particularly good results are achieved with partially incompatible blends in which at least one .of the resins is a polyester resin comprising the condensation polymerization product of ethylene glycol and terephthalc acid. Excellent results may be effected by blending, as a first component, polyester resins which are polymers of ethylene terephthalate containing ethylene sebacate or ethylene adipate, preferably in minor amounts. (These polymers are produced by the condensation polymerization of ethylene glycol, terephthalic acid and sebacic or adipic acid), with, as a second component, polyester resins described in U.S. Patent 2,965,613 to Milone et al. as copolyesters which contain at least 25% ethylene isophthalate and the remainder ethylene terephthalate. Polyester resins of this type are commercially available under the Vitel trademark, polyester resin Vitel PE207 being equivalent to the first component in the blend and polyester resins Vitel PE100 and Vitel PE200 being equivalent to the second component in the blend.

In addition, good results may be effected by blending the above mentioned copolyesters of U.S. Patent 2,965,613 containing at least 25% ethylene isophthalate with the copolyesters, also described in said patent which are lower in ethylene isophthalate, containing from to ethylene isophthalate and from 75% to 90% ethylene terephthalate. Also, by using solvents in which po1y(ethyl- =percent haze ene terephthalate) is soluble, poly(ethylene terephthalate) may be substituted for the copolyester of higher ethylene terephthalate content in the blend.

These polyester resins also give very good results when blended with other resins so as to be partially incompatible. For example, blends of the polyester resins of Patent No. 2,965,613 having the higher ethylene isophthalate-content polyester resins as a second component with acrylic ester polymers as the first component have been found to be very good embodiments of this invention. These acrylic ester polymers are preferably homopolymers and copolymers of esters of acrylic or methacrylic acids with lower alkyl alcohols. Likewise blends of the higher ethylene terephthalate-content polyester resins or the ethylene terephthalate-ethylene sebacate or adipate as a first component and styrene-butadiene rubber resins as a second component have been quite effective as have blends of acrylic ester copolymers and styrenebutadiene rubber resins.

The following examples will illustrate the practice of this invention:

Example 1 The following mixture is ball milled to a smooth consistency:

Parts by weight Lucite 2046 (a copolymer of n-butylmethacrylate and 50% isobutyl methacrylate) 7.3 Pliolite S-7 (a rubber resin consisting of a copolytrier of styrene and 30% butadiene) 1.8 Zinc oxide (Photox 801) 45.1 Toluene 41.0 Rose Bengal dye (a 0.12% solution in methanol) 6.0 Methylene Blue dye (a 0.12% solution in methanol) 2.0 Iosol Yellow dye (a 0.24% solution in toluene) 4.0

The mixture is then coated on white bond paper using a doctor roller and the coated paper is air dried to a dry film thickness of 0.5 mil.

For comparison purposes, two more sheets are prepared using the components, procedure and conditions set forth above except that in place of the blend of the two resins, there are respectively used 9.1 parts of Lucite 2046 alone in the first addition sheet and 9.1 parts of Pliolite S-7 in the second additional sheet.

Using the procedure and the apparatus described in U.S. Patent 3,052,539, particularly FIG. 5 thereof, each of the three sheets is negatively charged and exposed to a light image and then developed to form a copy. The sheet containing the blend of the two resins gives copies having images of equivalent intensity to those formed on the sheet containing Pliolite S7 alone but in almost onefourth the exposure time. Likewise, the sheet containing the blend gives images of equivalent intensity to those containing Lucite 2046 alone in less than one-half the exposure time.

In order to deter-mine the increased haze in films of the blend as compared with the films containing only the individual resins, compositions of the above resins in which the dyes and the zinc oxide have been eliminated are prepared. They have the following compositions:

Blend: Parts Lucite 2046 7.3 Pliolite S-7 1.8 Toluene 41.0

Resin:

Lucite 2046 9.1 Toluene 41.0 Pliolite S-7 9.1 Toluene 41.

Films, 1 mil in thickness (dry), are coated on clear colorless glass slides (46 mils in thickness). Then, using the GE. Spectrophotometer, the percentage of the total light which is transmitted through each coated slide is determined as well as the percentage of the total light which is scattered in each case. Then using the following formula:

Percent of the total light scattered Percent of the total light transmitted the percent haze is determined in case to be:

=Percent haze Percent haze Film of blend of Lucite 2046 and Pliolite S-7 11.90 Film of Pliolite S7 alone 0.78 Film of Lucite 2046 alone 0.50

This haze is also visible with the naked eye, giving the test film of the blend a milky white appearance while the films of the individual resins have no such appearance.

Example 2 Following the procedure of Example 1, a coated copying paper is prepared from the following composition:

Parts Copolyester of about 64% ethylene terephthalate As in Example 1, two additional sheets are prepared with 11.93 parts of the copolyester of ethylene terephthalate and ethylene sebacate alone and the copolyester of ethylene terephthalate and ethylene isophthalate alone respectively in place of the resin blend.

Using the copying process and apparatus of US. Patent 3,052,539, the sheet containing a coating of the blend gives copies having images of equivalent intensity to those formed on the sheets containing coating of either resin alone in about /a the exposure time.

Likewise, in order to determine the percent haze of films of the blend as compared to films of the component resins alone, films on glass slides are prepared of the blend and of the component resins individually by eliminating the zinc oxide and the dyes from the compositions of this example.

The following are the haze percentages:

Film of Percent haze Ethylene terephthalate-ethylene sebacate copolyester 1.34 Copolyester of ethylene terephthalate and ethylene isophthalate 0.49 Blend of the two resins 1.47

Example 3 Following the procedure of Example 1, a coated copying paper is prepared from the following composition:

Parts Copolyester of about 64% ethylene terephthalate and 36% ethylene sebacate 3.0 Pliolite S-7 (3. rubber resin consisting of a copolymer of styrene and butadiene) 6.0 Zinc oxide 35.8 Toluene 55.2

6 Film of Percent haze Copolyester 1.34 Pliolite S-7 0.87 The blend 13.50

The haze in the blend film is also markedly visible to the naked eye.

Example 4 Following the procedure of Example 1, a coated copying paper is prepared from the following composition:

Parts Poly(n'butylmethacrylate) 9.0

Copolyester of 53% ethylene terephthalate and 47% ethylene isophthalate 0.2 Zinc oxide 45.0 Rose Bengal dye (a 0.12% solution in methanol) 6.05 Iosol Yellow dye (a 0.24% solution in toluene) 4.0 Methylene Blue (a 0.12% solution in methanol) 1.9 Toluene 41.0

Also, as in Example 1, sheets are prepared with copolyester and polymethylmeth-acrylate alone in place of the resin blend. The sheets coated with compositions containing the blend give copies having images of equivalent intensity to those formed on the sheets containing coatings of either resin alone in about /2 the exposure time.

Likewise following the procedure of Example 1, the haze measurements of films of the blend are compared to films of polymethylmethacrylate alone and the copolyester alone as follows:

Film of- Percent haze Poly(n-butylmethacrylate) 0.5 Copolyester 0.49 Blend 12.9

The haze in the film of the blend is also markedly visible to the naked eye.

Example 5 Fol-lowing the procedure of Example 1, a coated copying paper is prepared from the following composition:

Parts Copolyester of 64% ethylene terephthalate and Also, as in Example 1, sheets are prepared with the copolyester and polymethylmethacrylate alone in place of the resin blend. The sheets coated with compositions containing the blend give copies having images of equivalent intensity to those formed on the sheets containing coatings of either resin alone in about the exposure time.

Likewise following the procedure of Example 1, the haze measurements of films of the blend are compared to films of the individual resins alone as follows:

Film of Percent haze Ethylene terephthalate-ethylene sebacate copolyester 1.34 Ethylene terephthalate-ethylene isophthalate copolye-ster 0.5 Blend 47.8

The haze .in the film of the blend is also markedly visible to the naked eye.

While there have been described what is at present considered to be the preferred embodiments of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed to cover all such changes and modifications as fall within the true spirit and scope of the invention.

I claim:

1. A recording element for electrostatic printing comprising a base sheet carrying a photoconductive insulating layer comprising zinc oxide suspended in an electrically insulating film-forming vehicle which comprises a blend of two partially incompatible resins in such proportions that said resins are partially incompatible with each other to the extent that a dry film of the blend alone deposited from an organic solution has a greater haze than films of equivalent thickness containing either of the resins alone in an amount equal to the total resin content of the blend while having an absence of macroscopic discontinuities, said blend of resin being of the group (A) a blend of polyesters, wherein one of the resins is a polyester comprising the condensation polymerization product of ethylene glycol and an acid component comprising at least 25% of the total acid compound weight of isophthalic acid and the remainder terephthalic acid, and the other resin is a polyester comprising the condensation polymerization product of ethylene glycol and an acid component comprising terephthalic acid and sebacic acid, (B) a blend wherein one of the resins is a copolyester of ethylene terephthalate and ethylene sebacate and the other resin is a copolymer of styrene and butadiene and (C) a blend wherein one of the resins is a copolymer of n-butylmethacrylate and isobutylmethacrylate and the other resin is a copolymer of styrene and butadiene.

2. The recording element of claim 1, wherein the se- -bacic acid in the second polyester constitutes a minor portion of the total acid content of said polyester.

3. A recording element according to claim 1 wherein one of the resins is .a polyester of from to ethylene isophthalate and from 50 to ethylene terephthalate, and the other resin is a copolyester of ethylene terephthalate .as a major component and ethylene sebacate as a minor component.

4. The recording element of claim 1 wherein one of the resins is a copolyester comprising ethylene terephthalate and ethylene isophthalate, at least one part of ethylene isophthalate being present in the copolyester for each 3 parts of ethylene terephthalate, and the other resin is poly n-butylmethacrylate) 5. The recording element of claim 1 wherein one of the resins is a copolyester of ethylene terephthalate and ethylene sebacate and the other resin is a copolymer of styrene and butadiene.

6. The recording element of claim 1 wherein one of the resins is a copolymer of n-butylmethacrylate and isobutylmethacryl-ate and the other resin is a copolymer of styrene and butadiene.

References Cited UNITED STATES PATENTS 2,959,481 11/1960 Kucera 961.8 2,990,279 6/1961 Crumley et al. 961.8 3,079,253 2/1963' Greig 961.8 3,121,006 2/1964 Middleton et a1. 961.5 X 3,132,941 5/1964 'Stahly et al. 961.8 3,152,895 10/1964 Tinker et al 961.8 3,159,483 12/1964 Behmenburg et al. 96-1.5 X 3,268,332 8/1966 Caruso et al. 96--1 NORMAN G. TORCHIN, Primary Examiner.

C. E. VAN HORN, Assistant Examiner. 

